Antenna arrays are increasingly used in electronic communications, including in the aerospace and the wireless telecommunications, for example. Antenna array test and calibration solutions are used to characterize the antenna arrays. Conventional solutions depend primarily on a vector network analyzer, which requires the device under test (DUT) including the antenna array, or antenna under test (AUT), to have radio frequency (RF) connectors, such as coaxial connectors, in order to perform the test and calibration. However, with the evolution of wireless communication technologies, antenna arrays with direct connections to (i.e., integrated with) RF transceivers of DUTs, and having no RF connectors, are becoming increasingly common. Overall performance of such a DUT presently must be tested “over-the-air,” since there is no place to connect a coaxial cable from the DUT and/or the antenna array to the test equipment.
Antenna characterization processes typically take place either at an outdoor test range or a chamber test range. The outdoor test ranges are used for antennas having a very long far field (e.g., greater than 5 m), rendering use of a chamber impractical. Chambers are used for sufficiently short far field measurements and/or near field measurements. The chambers may be anechoic chambers, for example, which are shielded chambers with walls covered in absorbing material that minimizes internal reflections, typically by several tens of decibels. Making near field measurements and transforming to obtain the far field can provide the far field radiation profile information in a smaller chamber.
The next generation of wireless communication systems, including handsets and corresponding infrastructure (e.g., base stations and backbone) is referred to as fifth generation or “5G.” 5G communication systems involve millimeter-wave frequency usage, compact phased array antennas, and significant electronic integration. Not only are transmitters and receivers of a DUT integrated into transceivers, but the transceivers are integrated with patch arrays, which have no traditional external connector from the radio electronics (e.g., handset or base station) to the transmit/receive (T/R) antenna. Instead, the entire radio, including antenna or antenna array, is a single indivisible unit referred to as an active antenna system (AAS). Of course, the AASs will still need testing for the usual characteristics, such as receiver sensitivity without and with interference present, total transmit power, error vector magnitude (EVM) of modulation formats, and antenna radiation pattern, for example. The parameters must be measured and studied in detail during product design phase, and characterization can be winnowed down during the manufacturing phase. However, the speed, accuracy and efficiency of testing are important to keep costs down and remain competitive.
The integrated (non-separable) nature of an AAS renders traditional transceiver testing methods unworkable. Traditionally, the antenna is disconnected, and all receiver and transmitter tests are performed by connecting test equipment to the radio's external connector. However, now there is no such connector on an AAS, as mentioned above. Further, the integrated nature of the 5G antenna introduces new challenges in testing the antenna itself. Conventional far field test chambers are large and expensive, so compact antenna test solutions, such as near field testing, are desirable. However, to apply Fourier transform methods to convert near field measurement data to far field radiation patterns, both amplitude and phase information are needed in the near field sampling. When the antenna can be disconnected, as in conventional systems, this is straightforward to achieve. That is, one can simply use a 2-port network analyzer with the AUT as Port 1 and a calibrated antenna or horn as Port 2. However, when the AUT is inseparable from the transceiver, phase information can be unreliable because the phase of the DUT's local oscillator (LO) is likely to drift relative to the phase of the test equipment's LO.
The speed of testing may also become an issue, which is not adequately addressed by proposed over-the-air (OTA) test solutions, for example, many of which involve mechanical scanning over two or more degrees of rotational freedom. For example, in conventional near field testing of non-AAS DUTs, a probe is raster scanned over X and Y degrees of freedom, which is already a slow process. When the DUT is an AAS, this method does not by itself produce reliable phase information, due to the LO phase drift, mentioned above. One may attempt so-called phaseless measurements by raster scanning in a parallel offset X-Y plane, and then using iterative algorithms, such as the Gerchberg-Saxton algorithm, to process amplitude information from the two planes, respectively, and infer the phase. However, this procedure more than doubles measurement time because the probe must be Z-translated to the new offset X-Y plane, and performance of the algorithm itself can take a long time to converge.
In conventional far field testing, the DUT must still be gimbaled over azimuth and elevation degrees of freedom, not over X and Y translation degrees of freedom. This rotational motion and its control are complicated, and may be even slower than the X-Y plane translation. In addition, parallel acquisition speedup by using multiple probe horns may not be straightforward. For example, if N horns are acquiring signals simultaneously, then when scanning a plane or a cylinder, there is a speed-up factor of N. But, when scanning a sphere (which is the desired far field geometry), one encounters redundant azimuth-elevation coordinate access. Thus, the speed-up factor is less than N.
With the advent of 5-bit to 6-bit amplitude and phase control of every antenna element in a DUT's patch array, the variety of possible radiation patterns for a 5G design is enormous. Multiply this by the number of carrier frequencies for testing, multiply again by 2× for both polarizations to test, and one has an enormous amount of data to acquire in order to test the DUT. Thus, it presently may take eight or more hours to test a single DUT array using conventional means.
Accordingly, there is a need for a large speed-up factor for obtaining far field measurements of DUTs with integrated antenna arrays, including phase correction.